This invention relates to high efficient aerodynamic lifting surfaces. More particularly, this invention relates to airfoils such as wings and control surfaces which are configured to reduce or minimize induced drag.
It has long been recognized in the art that the aerodynamic efficiency of a lift-producing airfoil such as an aircraft wing is affected not only by the profile drag of the airfoil, but by a drag component commonly called induced drag. This induced drag results from pressure on the upper and lower surfaces of the wing and, from the airflow direction caused by the lift producing wing-like structure. With respect to induced drag, this airflow is most significant at and near the ends or tips of the wings since the pressure differential produces airflow that is transverse to the stream of lift-producing air. Since induced drag is most significant at the wing tips, it has been recognized in the art that, considering a wing having an area S, the aerodynamic efficiency can be increased by increasing the span of the wing, i.e., maximizing the wing aspect ratio b.sup.2 /S, where b is the wing span.
Simply increasing the wing span to decrease the effect of induced drag and thereby increase aerodynamic efficiency is subject to structural constraints which limit the efficiency increases that can be attained. For example, to achieve structural integrity of the aircraft, the thickness of the wing must be increased as the wing is made longer. Such an increase in thickness increases the weight of the wing. Not only does the increase in profile which results form increasing wing aspect ratio drag detract from efficiency gains effected by a higher wing aspect ratio, but a point is eventually reached where the increase in profile drag totally offsets the benefits attained by a higher aspect ratio. Further, the attendant increase in wing weight often means that the wing must be operated at a higher coefficient of lift C.sub.L in order to provide the desired aircraft performance. Since the induced drag term is not only an inverse function of the wing aspect ratio, but is also directly proportional to C.sub.L.sup.2, it can again be seen that high aerodynamic efficiency cannot be attained simply by increasing wing aspect ratio.
Because of the limitations associated with planar wings of increased aspect ratio, various other means of decreasing induced drag have been proposed. For example, in U.S. Pat. No. 1,724,110, which issued to E. G. Reid, on Aug. 13, 1929, relatively thin fins or shields that extend streamwise along the wings and project outwardly from the wing surfaces to prevent or impede transverse flow along the wing surfaces are utilized. In this respect, endplates that project orthogonally above and below the wing tips are disclosed along with similarly constructed shields or plates that can be utilized at various locations along the wingspan.
Additionally, in U.S. Pat. No. 3,270,988, which issued to C. D. Cone, Jr., on Sept. 6, 1966, various other nonplanar wing designs are described which increase the effective aspect ratio of the wing to thereby attain higher aerodynamic efficiency. More specifically, although the disclosure of the Cone patent is primarily addressed to apparatus and methods for analyzing nonplanar wing configurations with respect to ascertaining the effective aspect ratio and thereby determine the aerodynamic efficiency of the configuration, various nonplanar wing geometries are suggested within this patent. Among these wing configurations are tubular sections of circular and elliptical cross-sectional geometry which are mounted at the tips of the planar wing with the axis of such tubular sections being substantially parallel to the flow direction of the free stream air and, a nonplanar wing which is divided at the tip into a number of branches that extend arcuately upward such that the wing terminus is effectively a number of "winglets" of different curvature which radiate from a planar wing.
Although endplates and other structures that have been previously proposed are generally satisfactory with respect to low speed aircraft, various disadvantages and drawbacks have prevented such apparatus from being incorporated in the design of modern high-speed aircraft. First, since modern high-speed aircraft are generally designed to operate a relatively high coefficients of lift, the profile drag associated with endplates or other nonplanar wing geometries is of even greater importance that it is with respect to low speed aircraft. Further, since such endplates are located in and influence the flow field of the wing, interference drag is created which at least partially cancels the benefits achieved by decreasing induced drag. Such interference drag often increases and becomes more of a problem in high speed aircraft since flow separation can easily occur at the boundary or transition between the wing and endplate structure. Additionally, since such high speed aircraft generally cruise at transonic flight velocities, shock waves can be induced by such boundary ior transition regions wich result in compressibility drag that can completely offset any increase in aerodynamic efficiency that is effected under low speed conditions. Even further, and expecially with respect to nonplanar configurations in which the structure for reducing induced drag also provides lift, the spanwise pressure distribution along the wing can be affected to greatly increased wing bending moments. For example, not only does the weight increase that is often brought about by the use of endplates increase the wing bending moment, but if the section of the wing which reduces induced drag also produces lift, such as the branched winglet arrangement in the patent to Cone, further increases in bending moments occur.
Because of the above-mentioned drawbacks and others, there has been relatively little interest in applying endplates (or, as they have come to be called, tip fins or winglets) to modern high speed aircraft until the potential exhaustion of petroleum resources became apparent and, as a consequence, the cost of aircraft fuel increased. Recognizing that an increase in aircraft operating efficiency not only conserves energy resources, but is important in providing aircraft that can be operated economically, those skilled in the art have thus begun to consider the design of more efficient aircraft engine and aircraft structure.
In this respect, it should be noted that both the military and commercial operators of high speed aircraft presently possess a substantial number of such aircraft and a large number are also currently in production. Thus, not only are designs necessary and desirable for a new generation of highly efficient aircraft, but designs are required for retrofitting to existing aircraft and for incorporation in aircraft currently being produced without requiring major design changes.
Accordingly, it is an object of this invention to provide a nonplanar wing configuration which exhibits high aerodynamic efficiency on both low speed and high speed flight of the aircraft.
It is another object of this invention to provide a wing configuration wherein induced drag is reduced relative to a conventional wing of the same aspect ratio and wherein a wing tip fin produces minimal profile drag and causes minimal interference and compressibility drag.
It is yet another object of this invention to provide a wing tip fin for reducing induced drag wherein a tip fin is also configurated for causing minimal increase in wing bending moment normally attendant to adding structure to the outboard portions of the wing.
Even further, it is an object of this invention to provide a wing tip fin of the above-described type which is readily incorporated in present aircraft designs and readily retrofittable to existing high speed aircraft.